Why is this research needed?

Carbon dioxide (CO2) geosequestration represents one of the most promising options for reducing atmospheric emissions of CO2. It has been proposed as one solution to global climate change caused by heat-trapping of anthropogenic gases in the atmosphere. CO2 can be stored in aquifers or depleted oil (or gas) reservoirs. However, caprock integrity ascertained based on the petrophysical and geomechanical properties of caprock, as well as CO2 properties and degree of purity, is vital to ensure safe and sustainable storage of CO2. Caprock is any impermeable or low permeability formation that may trap gas, oil or water, preventing it from migrating to the surface. Originally, most caprocks have high mechanical strength, high stiffness, low compressibility and very low permeability (Smith et al., 2009). However, during petroleum depletion, as well as CO2 injection and storage, caprock undergoes stresses and strains which impact its integrity. Thus, to properly evaluate caprock integrity, certain criteria including sealing capacity (sealing number), mechanical stability (stability number) and factor of safety (based on rock theories of failure) must be set to reflect effect of rock geomechanical and petrophysical properties changes, and CO2 injection and storage operational conditions on caprock integrity.

CO2 injection wells are exposed to low temperature and strong temperature variations, which threatens well integrity and the integrity of the reservoir-caprock system. Freezing of pore or connate water in the reservoir or caprock has the potential to crack or fracture the formation. A solution could have been to have temperatures above the freezing point of the brine (or pore water) at that particular depth but, in that case, heating can result in radial or thermal fracturing of the formation resulting in further damage to the reservoir-caprock system. The porosity of caprock increases at low temperature and reduces as temperature increases when CO2 is injected. Self-healing of rock may occur where CO2 reacts with impurities in the storage reservoir. However, the chemical reaction between the injected CO2 and rock samples might vary for different rock mineralogy and proportion of impurities in the CO2.

What is this research investigating?

The main objectives of this research project are:

• To conduct a laboratory-scale experiment to investigate the degree of purity of CO2 required to mitigate caprock embrittlement in cold temperature conditions
• To conduct a laboratory-scale experiment to determine the impact of treating caprock samples with CO2 associated gases on the caprock breakthrough pressures
• To conduct a laboratory-scale experiment and develop a computer simulation model to determine optimum CO2 injection temperature to assure caprock integrity and minimize the possibility of stress concentration in the reservoir-caprock system
• To conduct a laboratory-scale experiment and develop a computer simulation model to investigate the impact of CO2 injection pressure on caprock integrity based on amount of deformation and stress developed in the reservoir-caprock system
• To develop a machine learning model to evaluate caprock integrity based on the sealing capacity and mechanical stability of the caprock samples, as well as factor of safety of the caprock samples considered based on the stress developed during the experiments

What does the research hope to achieve?

A promising solution to mitigate caprock embrittlement and thermal fracturing in low (cold) temperature CO2 storage sites is to initiate a chemical interaction between the caprock and CO2 by introducing certain gases that are proven friendly when in contact with CO2. This could help to reduce permeability and porosity of the caprock induced by frozen pore or connate water in low temperature formation, self-heal possible thermal fractures created in the rock due to strong temperature variations, heal caprock embrittlement resulting from the cold temperature, and increase the caprock breakthrough pressure and storage life of the sites through treatment by the chemical interactions between CO2, injected gases and the caprock.

So far, the identified potential solutions have not been investigated to mitigate caprock embrittlement in cold formations. It is against this background that the researcher has conceived the idea to investigate the impact of the degree of purity of the CO2 injected in CO2 storage sites on caprock integrity to fill the identified gap. Therefore, the aim of this study is to evaluate caprock integrity in cold temperature formation for different degree of purity of CO2 stored in depleted oil and gas reservoirs.